sure you can do it all...(well, the MTB race might be a tall order). Just get yourself an older touring frame or a cross frame.

I only have a single bike, 3 sets of wheels (ok, two, was recently in an accident). I generally run 27x1 1/8 for general puttin; run 700x38 for trails. Run 700x23-or-so and swap to drops when I'm in go-fast mode.

Its a great do-it-all setup, but it probably isnt optimal if youre racing competitively. Then again, if you're competively racing, you wouldnt be askin....